Interpretation ID: nht75-3.9
DATE: 11/28/75
FROM: AUTHOR UNAVAILABLE; Frank A. Berndt; NHTSA
TO: Ford Motor Company
TITLE: FMVSS INTERPRETATION
TEXT: This is in response to your letter of September 24, 1975, in which you ask whether it is permissible to test certain 1978 vehicles for compliance with Standard No. 301, Fuel System Integrity, with open vapor vent tube pressure relief valves.
The Federal motor vehicle safety standards do not specify the tests which you must perform. They do specify the conditions and procedures under which the National Highway Traffic Safety Administration (NHTSA) will conduct its compliance testing. S7.1.1 and S7.1.2 of Standard No. 301 specify that the vehicle's fuel system shall contain Stoddard solvent rather than fuel and, by implication, that the engine shall not be running. If, as you indicate, one consequence of the engine's not running is that a certain pressure relief valve in the vapor vent tube is closed, then that valve must remain closed during the NHTSA's compliance testing; the existing standard could not be interpreted otherwise. Although in an actual collision any rollover would probably occur immediately after the initial impact, in some accidents vehicle occupants would be trapped for some period of time after rollover. Therefore, we do not consider that this interpretation creates, as you suggest, an artificial condition by subjecting the fuel tank to a potential vapor pressure build-up during preparation for the rollover test.
Sincerely,
ATTACH.
September 24, 1975
James B. Gregory -- Administrator, National Highway Traffic Safety Administration
Dear Dr. Gregory:
Re: Request for Interpretation in Relation to Conducting FMVSS 301-75 Tests with Proposed 1978 Evaporative Emission System
In preparing for compliance with 1978 California SHED evaporative emission requirements (presently under consideration by EPA for application in all areas), Ford Motor Company (Ford) is considering modifications to the fuel tank vapor venting system. At this time, Ford's primary design direction for certain of its 1978 vehicles is to incorporate a pressure relief valve in the vapor vent tube between the fuel tank vapor separator/rollover valve and the carbon canister (see Attachment).
When the engine is operating, this pressure relief valve will be open and the fuel tank will vent in a normal manner through the carbon canister. When testing according to the procedure set forth in FMVSS 301-75, the pressure relief valve would be closed, contrary to its normal open position when the engine is operating, and such closure would prevent venting of vapor from the fuel tank into the carbon canister. As a result, vapor pressure could build up within the fuel tank during FMVSS 301-75 testing, and indeed would do so if the fuel tank were exposed to elevated ambient temperatures during the substantial time period (in some instances several hours) required to ready the test vehicle for the rollover test following an impact test. If such pressure built up, it (taken together with the hydrostatic pressure of the Stoddard solvent in the tank) might force open the vacuum/pressure valve in the fuel tank cap and permit leakage through that valve (cap).
Such leakage would not occur in an accident involving a rollover because, in such a situation, vapor pressure would not build up in the fuel tank, and therefore, the vacuum/pressure valve in the cap would not open. Any rollover of the vehicle that may occur would happen immediately after the collision, before vapor pressure build-up. Hence the vacuum/pressure valve (cap) would remain closed after rollover. More particularly, just before the accident, the vapor vent tube pressure relief valve would be open, preventing vapor pressure build-up.
Accordingly, Ford proposes to perform tests relating to compliance with FMVSS 301-75 with the vapor vent tube pressure relief valve open, the better to simulate actual usage conditions. Before testing in this manner, Ford would appreciate receiving assurance from the Administration that it regards as appropriate the maintaining open of this pressure relief valve during rollover tests, and would conduct in that manner its rollover tests of those Ford vehicles equipped with such a valve.
As you are undoubtedly aware, in addition to design and development work, a test program required to obtain emission certification covers a period of many months. Therefore, since Ford has only a limited time in which to develop a 1978 evaporative emission system complying with the SHED requirements, an early response to this letter is urgently requested.
Respectfully submitted,
J. C. Eckhold -- Director, Automotive Safety Office, FORD MOTOR COMPANY
ATTACH.
PROPOSED 1978 EVAPORATIVE EMISSION SYSTEM
(Graphics omitted)
Automotive Safety Office September 22, 1975